Gallium nitride compound semiconductor elements are attracting customers' attention as semiconductor devices for high frequency and high power because they have their high breakdown electric field and high electron mobility of substantially the same level as that of gallium arsenide semiconductor. For example, a prior art Schottky barrier diode utilizing gallium nitride, comprises a sapphire base plate, a compound semiconductor region of gallium nitride prepared by forming a buffer layer of a single or a plurality of laminated gallium nitride (GaN) compound semiconductor layers on the base plate and further forming undoped gallium nitride (GaN) and undoped aluminum gallium nitride (AlGaN) layers on the buffer layer, a Schottky barrier electrode formed on a top surface of the compound semiconductor region, and an ohmic electrode. The Schottky barrier electrode forms a Schottky barrier on a boundary surface of AlGaN layer, and ohmic electrode is in contact to AlGaN layer with low resistance. By way of example, U.S. 2006/0108659 A1 shows a gallium nitride compound semiconductor element having a Schottky barrier electrode.
Recently, attempts have been made to utilize silicon base plates in lieu of sapphire base plates. A gallium nitride compound semiconductor formed on a silicon base plate is advantageous because it can easily be mechanically processed such as dicing and manufactured in lower cost compared to gallium nitride compound semiconductor on a sapphire base plate.
On the other hand, gallium nitride compound semiconductor shows a drawback of fluctuation in electric potential of electrically conductive base plate of such as silicon depending on change in an operating condition of the device, thereby undesirably making electric property of the device unstable. To solve this problem, an effort has also been devoted to electrically connect an electrode formed on a bottom surface of base plate to an anode electrode, thereby clamping the base plate at electric potential of anode electrode.
A semiconductor element of the structure connecting an electrode of base plate to an anode electrode has still a deficiency in that it cannot sufficiently increase withstand voltage because a leakage current flows in the thickness or vertical direction along a side or creepage surface of the element between compound semiconductor region and base plate when voltage is applied between base plate and anode electrode formed on a top surface of gallium nitride compound semiconductor region. To solve this problem, a proposition has been offered to thicken compound semiconductor region to reduce leakage current for improvement in withstand voltage. However, it is very difficult to epitaxially grow a thicker compound semiconductor region of good crystallization on a silicon base plate.
In another aspect, it was thought that a well-known silicon-on-insulator (SOI) base plate is used to interpose an insulating film between a silicon base plate and a gallium nitride compound semiconductor region to reduce leakage current for improved withstand voltage. However, even in this element structure, electric discharge may occur between side surfaces of compound semiconductor region or electrically conductive film and base plate, and it can hardly establish its stable and high withstand voltage property. Also, a high electron mobility transistor (HEMT) has the structure which comprises a silicon base plate, a gallium nitride compound semiconductor region layered on silicon base plate, and source, drain and gate electrodes formed on gallium nitride compound semiconductor region wherein source or drain electrode is electrically connected to silicon base plate, however, for similar reasons as above-mentioned, HEMT also disadvantageously cannot establish its stable and high withstand voltage property. Moreover, a similar difficulty arises when using electric conductive base plates other than silicon base plate.
In this way, it was difficult to establish a high withstand voltage in a prior art compound semiconductor element which electrically connects a surface electrode laminated on conductive base plate with conductive base plate.
An object of the present invention is to provide a compound semiconductor element which indicates a high withstand voltage property although it comprises a conductive base plate, a compound semiconductor region laminated on the conductive base plate and a surface electrode formed on the compound semiconductor region and electrically connected to the base plate.